The invention relates generally to the area of numerically controlled machines. Specifically, the invention provides a numerically controlled machine with a programmably controlled sensing element for detecting the location and position of reference characteristics on a workpiece. This capability enables the part programmer to cope with set-up and part-to-part variations, thereby minimizing operator intervention and significantly improving productivity.
While the invention of numerical control has greatly improved the speed and reliability with which many workpieces may be machined, there are many workpieces which still require a time consuming intervention by a skilled operator in the machining process.
The machining process begins with a part programmer who prepares a machining program based on drawings of a finished part. In preparing the program, the programmer defines a particular location of the workpiece on the machine as well as particular dimensional reference surfaces of the unmachined part. The programmer then prepares a machining program comprised of blocks of instructions and data which is defined relative to a programmer created program coordinate system. In operation, a control connected to the machine generates command signals in response to the machining program to move a cutting tool relative to the workpiece thereby machining the workpiece to the desired finished part.
However, being machining, the machine operator must first position the part within the machine coordinate system to a location defined by the part programmer. Typically, the machine coordinate system is represented by three mutually perpendicular linear axes of motion. After positioning the part, the machine operator manually feeds the toolholder close to designated reference surfaces on the part or its fixture; and then, using a hand held gage and progressively smaller jogs, the operator positions the toolholder as precisely as humanly possible close to the workpiece. When the axes are in position, the operator operates a position set push button which establishes the located workpiece surface as having a specified program coordinate. The procedure of precisely defining the location of the workpiece within the machine based coordinate system can take from ten to fifteen minutes.
Many large rough castings have significant dimensional variations from what was assumed by the programmer. For example, the location of holes and bosses may vary up to 1/2 inch from a nominal position from casting to casting. Therefore, before the holes or surfaces are machined, the operator must realign the dimensional coordinates of the program with the actual location of the elements on the workpiece.
To address this problem, the numerical control contains an offset feature which provides a program selectable coordinate offset to shift all dimensions within a portion of the program by an operator entered increment in each of the major linear machine axes. Therefore, the operator, following the programmer's instructions, measures the distances from a reference to the holes or bosses. He manually enters the difference between these distances and the programmer's published distances into preselected offsets. Since a large part may require a great many offsets, the loss in productivity is cumulative. The above manual operations not only require a relatively high skill level, but in effect, reduces a highly sophisticated and expensive numerically controlled machine into a hand operated machine.
One object of the claimed invention is to eliminate the necessity of the operator interrupting the machining cycle to define the exact location of the workpiece and the position of reference characteristics on the workpiece.
Another object of the invention is to provide a workpiece measuring system for measuring the position of a plurality of points on a reference surface and to determine maximum, minimum and average values of the measurements. The average values may be used to locate the center of a rough opening, hole, or boss or be used to find the best fit on a rough surface. The maximum value may be used to define the clearance from a rough surface, and the minimum value may be used to establish the minimum amount of material to be removed to clean up a rough surface.
A further object of the invention is to use the measured position of a surface to calculate an offset value from a programmed definition of the surface.
The invention further provides a test of measured values and offsets against maxima and minima defined by the programmer to guarantee the part is within tolerance.
A final object of the invention is to use a cutting tool and torque measuring circuit to accomplish the surface sensing and accurately measure the workpiece location and workpiece surface positions.